The DK7ZB designs I plan to use give a 50 ohm impedance at the center of
a single split driven element. A small amount of gain is sacrificed to
accomplish that. I modeled the designs in EZNEC+ and they appear to be
spot on. In addition, quite a few VHF/UHF enthusiasts in Europe have
built antennas per his designs and verified the 50 ohm feedpoint impedance.
So all I really need is a common mode choke, and I will probably use the
clamp-on ferrites that K9YC suggests.
Thanks es 73,
Dave AB7E
On 6/28/2019 7:33 PM, Glenn Pritchard wrote:
A folded driven element is inherently 200/300 ohms, if you take the
Sinclair or Comprod antennas there is a 96 ohm transference within the
element when they talk about open dipole (feed point open) or closed dipole
as in the phasing. I made a 7 element yagi with a folded dipole driven
element from a 210-C1, designed the antenna around the driven element.
With a yagi, without this arrangement you have to take the inherent 300
ohms down to 53 ohms, hence the balun.
I did a LOT of work with similar antennas when I was with CNCP
Telecommunications and Unitel here in Canada.
Glenn, VA7UO
On Fri, Jun 28, 2019 at 5:47 PM jimlux <jimlux@earthlink.net> wrote:
On 6/28/19 11:50 AM, David Gilbert wrote:
I've never been much interested in VHF/UHF, but with the current level
of solar flux and the summer static I'm thinking of giving it a try
using homebrew antennas based upon DK7ZB designs. For 6m I'll probably
use a coaxial sleeve as a common mode choke (unless somebody can suggest
something better/simpler), but for 2m/70cm I'm planning to build a dual
band yagi with a common feedline and I'm not sure a sleeve would work. I
guess it might, since 1/4 wave and 3/4 wave (432/144 = 3.00) accomplish
pretty much the same thing, but I'd like some input from VHF/UHF hams
who would know better than I.
And if a sleeve balun is indeed the way to go, what is the best way to
fabricate one at UHF? As W8JI points out, coax jackets aren't
necessarily a low loss dielectric at UHF.
Thanks much for any suggestions.
A few ferrite beads/toroids of the appropriate material will suppress
current on the outside of the coax, which is what you want.
Use Jim's RFI cookbook, but rather than all those HF materials like #32,
pick a material with good properties at VHF/UHF (and I'll bet those
omnipresent ferrites used for EMI/EMC of computers are a likely choice).
Those little 75:300 ohm transformers for TV use are another possibility,
but it's a 2:1 turns ratio inside - so maybe if your driven element on
your yagi is a folded dipole it would work. TV is 50MHz to 800 MHz, so
covering 144 and 440 is easy.
Most Yagis have low feedpoint impedances in their native state - a
folded driven element might be a direct match to 50 ohms.
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